1. Field of the Invention
The present invention relates to a direct sequence code division multiple access (DS-CDMA) transmission method carrying out multiple access using spread spectrum in mobile communications, and particularly to a DS-CDMA transmission method that carries out code multiplexing of multiple code channels.
2. Description of Related Art
Recently, intensive research and development of the DS-CDMA system have been made as one of the next generation mobile communication systems. The DS-CDMA transmission system carries out communications between multiple users using the same frequency band, and individual users are identified by a spreading code properly assigned to each user.
The DS-CDMA system has advantages over the frequency division multiple access or time division multiple access in that it can increase the capacity in terms of the number of simultaneous subscribers within the same available frequency band, and is suitable for high speed signal transmission because it transmits information signals after spreading them into wideband signals.
In the mobile communication environment, straight paths between a base station and mobile stations are seldom unobstructed, thereby constituting multipath propagation. As a result, the received signal undergoes Rayleigh fading. In Rayleigh fading, the amplitude of a received signal has Rayleigh distribution, and its phase has a uniform distribution. It is necessary for a receiver to estimate the randomly varying phase of the received signal to carry out coherent detection which is more efficient than differential detection. One of the methods for estimating the received phase is implemented by inserting pilot symbols of a known pattern into information symbols at fixed intervals, and by estimating the received phase of each information symbol on the basis of the received phases estimated using the pilot symbols. In this case, the pilot symbols must be inserted at every time interval during which the phase fluctuation due to fading is nearly negligible.
There are mainly two methods for implementing high bit rate signal transmission in the DS-CDMA system: (1) a method which varies a spreading factor (processing gain) in accordance with the transmission information rate; and (2) a code multiplexing method which multiplexes multiple channels each having a fundamental information rate. Here, we will consider the second method.
FIG. 16 shows a conventional channel structure when carrying out the absolute coherent detection which makes the channel (amplitude and phase) estimation using the pilot symbols as mentioned above. In this figure, N denotes the number of code channels (the number of code multiplexing). Each code channel is spread using a short code (SC-1, . . . , SC-N) with a period equal to that of information symbol, and is further spread using a spreading code referred to as a long code (LC-Y) with a period much longer than that of the common information symbol. The short codes serve to identify the individual code channels, and the long code serves to distinguish a user from the other simultaneous users in the same cell in reverse link channels, and from the other simultaneous users in the other cells in forward link channels. FIG. 17 shows a frame structure of a single code channel transmission.
The conventional system has the following problems:
(1) The pilot symbols, which are inserted into each code channel as shown in FIG. 16, are spread by the same spreading code assigned to each code channel for spreading data symbols. This results in some cross-correlation between the multiplexed code channels, which degrades the accuracy of the channel estimation by the pilot symbols. In other words, the conventional DS-CDMA code multiplexing method has a problem in that the accuracy of the channel estimation using the pilot symbols degrades owing to the cross-correlation between other code channels because the pilot symbols of respective code channels are spread using different spreading codes. The degradation is remarkable when the received signal power per path decreases under the multipath environment.
(2) The pilot symbols, which are inserted at the same positions in frames in the respective code channels as shown in FIG. 18A, are used to estimate the received phases for each code channel so as to obtain the transfer function of the channel on information data sequence by means of interpolation at the insertion intervals of the pilot symbols as shown in FIG. 18B. This results in a problem in that the accuracy of the channel estimation degrades with an increase in the fading fluctuation speed. In addition, since the transmission power control is also carried out at the insertion intervals of the pilot symbols by measuring the received signal power at the positions of the pilot symbols which are inserted in the same positions in the frames of respective code channels as shown in FIG. 19, there is another problem in that the accuracy of the transmission power control also degrades with an increase in the fading fluctuation speed.